The present invention relates generally to pulse oximetry, and more particularly to pulse rate and blood analyte level estimation using cepstral domain processing of plethysmographic signals.
Current pulse oximeters obtain two signals derived from the attenuation of red and infrared light signals as they are passed through a patient tissue site, typically a finger. A number of processing methods have been developed in the industry in both time and frequency domains to obtain both pulse rate information and the oxygen content (SpO2) level of the arterial blood from the attenuated red and infrared light signals. The attenuated red and infrared signals show a pulsing waveform that is related to the heart rate of the patient. These time domain signals, usually after some bandpass filtering, are used for display of the pulse cycle and are known as plethysmographic signals. Prior techniques for pulse-rate estimation have mostly operated in the time domain and have used peak picking and analysis to derive a pulse rate. Time domain measures can respond quickly to pulse rate changes, but the presence of moderate motion and/or low amplitude pulses pose problems for accurate peak picking. Processing in the frequency or spectral domain has also been used and this requires a longer sample of the waveform to generate a pulse estimate. Also, identification of the predominant spectral peak produced by the pulse can be problematic in the presence of motion artifacts.
Accordingly, the present invention provides for processing of plethysmographic signals via the cepstral domain to enhance the determination of patient physiological condition related information such as patient pulse rate and SPO2 level information from plethysmographic signals, especially when motion artifacts are present in the plethysmographic signals. In accordance with the present invention, plethysmographic signals (e.g., attenuated red and infrared signals) are sampled and transformed into the cepstral domain, via, for example, a logarithmic like transform sandwiched between two forward Fourier transforms. Peaks in the cepstral domain are related primarily to the pulse rate. Identification of the pulse generated cepstral domain peak allows for pulse estimation in the presence of moderate motion artifacts. The cepstral information also allows for adaptive filtering of the input plethysmographic signals to remove noise and artifacts. The relative magnitudes of the cepstral peaks for both red and infrared signals in conjunction with an estimate of the DC levels of the red and infrared signals also allows for measurement of blood analyte (e.g., SPO2) levels of the blood.
According to one aspect of the present invention, a method of processing at least first and second time domain plethysmographic signals (e.g., red and infrared plethysmographic signals) obtained from a patient includes the steps of performing a Fourier transformation on the first time domain plethysmographic signal to transform the first plethysmographic signal into a first frequency domain plethysmographic signal and performing a Fourier transformation on the second time domain plethysmographic signal to transform the second plethysmographic signal into a second frequency domain plethysmographic signal. In this regard, the Fourier transformations may be fast Fourier transforms. A first power spectrum is computed from the first frequency domain plethysmographic signal and a second power spectrum is computed from the second frequency domain plethysmographic signal. A Fourier transformation is performed on the first power spectrum to transform the first power spectrum into a first cepstrum and a Fourier transformation is performed on the second power spectrum to transform the second power spectrum into a second cepstrum. In this regard, the Fourier transformations may be fast Fourier transforms. The first and second cepstrums are then examined to obtain information therefrom relating to a physiological condition of the patient.
The physiological condition of the patient may, for example, be the patient""s pulse rate. In this regard, the first and second cepstrums may be examined to identify peaks in the first and second cepstrums associated with the pulse rate of the patient, and the pulse rate of the patient may be estimated based on the locations of the identified peaks in the first and second cepstrums.
The physiological condition of the patient may also, for example, be the patient""s SPO2 level. In this regard, DC levels of the first and second power spectrums may be determined, AC levels of the first and second time domain plethysmographic signals may be determined from the identified peaks in the first and second cepstrums, and a value correlated with a blood analyte level (e.g., SPO2 level) of the patient may be computed from the DC values of the first and second power spectrums and the AC levels of the first and second time domain plethysmographic signals.
According to another aspect of the present invention, a method of determining a pulse rate of a patient from at least one time domain plethysmographic signal obtained from the patient includes the step of obtaining a time domain based estimate of the pulse rate of the patient from the time domain plethysmographic signal. The time domain plethysmographic signal is transformed to a spectral domain plethysmographic signal and a spectral domain based estimate of the pulse rate of the patient is obtained from the spectral domain plethysmographic signal. The spectral domain plethysmographic signal is transformed to a cepstral domain plethysmographic signal and a cepstral domain based estimate of the pulse rate of the patient is obtained from the cepstral domain plethysmographic signal. A best estimate of the pulse rate of the patient is then determined based on at least the time, spectral, and cepstral domain based estimates of the pulse rate of the patient.
According to one more aspect of the present invention, a pulse oximeter includes first and second optical signal sources operable to emit optical signals characterized by first and second wavelengths (e.g., red and infrared), respectively. The pulse oximeter also includes a drive system, a detector, a digital sampler (e.g., an analog-to-digital converter), and a digital processor. The drive system is operable to cause operation of the first and second optical signal sources such that each optical signal source emits first and second optical signals, respectively, in accordance with a multiplexing method. The detector is operable to receive the first and second optical signals after the first and second optical signals are attenuated by a patient tissue site of a patient. The detector is also operable to provide an analog detector output signal representative of the attenuated first and second optical signals. The digital sampler is operable to sample the analog detector output signal at a desired sampling rate and output a digital signal having a series of sample values representative of the attenuated first and second optical signals. The digital processor is enabled to demultiplex the series of sample values into first and second time domain plethysmographic signals, transform the first and second time domain plethysmographic signals into first and second spectral domain signals, transform the first and second spectral domain plethysmographic signals into first and second cepstral domain plethysmographic signals, and examine the first and second cepstral domain plethysmographic signals to obtain information therefrom relating to a physiological condition of the patient, such as the patient""s pulse rate or SPO2 level.
According to a further aspect of the present invention, a pulse arbitration method for use in determining a fundamental pulse frequency (or pulse rate) of a patient from multiple signal domains (e.g., time, energy, log, and cepstral) associated with at least one time domain plethysmographic signal obtained from the patient includes the step of transforming the time domain plethysmographic signal to a spectral domain plethysmographic signal. The spectral domain plethysmographic signal is transformed to a cepstral domain plethysmographic signal. The transformations to the spectral and cepstral domains may, for example, be accomplished via Fourier transformation operations. The spectral and cepstral domain plethysmographic signals are examined to identify corresponding spectral and cepstral domain plethysmographic signal peaks. The identified corresponding spectral and cepstral domain plethysmographic signal peaks are then used to select the fundamental pulse frequency from among a plurality of possible candidates for the fundamental pulse frequency of the patient. Possible candidates for the fundamental pulse frequency of the patient may, for example, be obtained from the time, spectral, and/or cepstral domain plethysmographic signals as well as from a filtered time domain plethysmographic signal and/or a log scaled spectral domain plethysmographic signal.
Cepstral domain processing of plethysmographic signals offers several advantages for pulse-rate identification. For example, the log-like transform acts to suppress weaker noise components making peak identification easier, and the cepstral peak is primarily generated via the harmonic components of the pulse so that noise energy surrounding this xe2x80x9cfundamentalxe2x80x9d pulse frequency does not adversely effect pulse frequency identification.